1. Field of the Invention
The present invention relates to an image forming apparatus having a function of correcting the density unevenness in the conveying direction, which is caused on the recording medium, and a density unevenness correcting method.
2. Description of Related Art
In an image forming apparatus for printing an image on a recording sheet, the density unevenness is caused due to various types of factors. In order to suppress the density unevenness, the following correcting process is generally carried out. In the correcting process, a test image is printed on the recording sheet to measure the density by optically reading the test image, and the correction data is prepared so as to cancel the measured density unevenness. Then, by using the correction data, the image data to be printed is corrected.
The density unevenness is caused in the sub-scanning direction which is the conveying direction of the recording sheet, in addition to the main scanning direction which is perpendicular to the conveying direction of the recording sheet. Therefore, in order to precisely correct the density unevenness, it is desirable to correct not only the density unevenness in the main scanning direction, but also the density unevenness in the sub-scanning direction.
In the density unevenness caused in the sub-scanning direction, the density unevenness is caused as the periodic unevenness of the rotary member, such as the developing sleeve, the photoconductive drum, and the like. In order to correct the above density unevenness, it is necessary to correct the density unevenness so as to match with the period and the phase of the rotary member.
Because the phase of the rotary member is asynchronous with the timing of conveying the head of the sheet, or the like, it is necessary to obtain the special signal or the like to synchronize the phase of the rotary member with the head of the sheet. As an example, there is a method in which the home position signal (hereinafter, referred to as “HP signal”) of the rotary member is obtained. The HP signal is a signal for informing that the rotary member is placed in the reference position, and is generated when the rotary member passes the reference position in every one rotation.
As an example for generating the HP signal, the plate having a half moon shape is attached to the rotation shaft, and the plate is read by a photoelectric sensor. The HP signal output by the photoelectric sensor is a rectangular wave in which one rotation of the rotary member is defined as one period. Therefore, in case that the timing at which the HP signal rises is monitored, it is possible to recognize the timing at which the rotary member is placed in the specific phase (the reference position) in one rotation.
In order to correct the periodic density unevenness caused due to the rotary member by matching with the period and the phase of the rotary member, it is necessary to prepare the correction data for one period of the rotary member. As a method for preparing the correction data, the following method has been generally adopted. In the method, a pattern having the uniform density is drawn in the sub-scanning direction on the recording medium, and the density unevenness is grasped by measuring the density of the above pattern with a density sensor. Then, the correction data for cancelling the measured density unevenness is prepared.
The density data obtained by drawing the pattern on the recording medium, which corresponds to one period of the rotary member to be corrected, and by measuring the pattern with the density sensor, includes the density unevenness caused due to the members other than the rotary member to be corrected (each rotary member having the different period from that of the rotary member to be corrected, and the like). The above density unevenness becomes a noise when the periodic density unevenness caused due to the rotary member to be corrected (hereinafter, also referred to as “periodic unevenness”) is precisely obtained.
The long pattern corresponding to a plurality of periods of the rotary member to be corrected is drawn in the sub-scanning direction, and the density data obtained by measuring the density of the pattern is divided into a plurality of the density data for one period of the rotary member to be corrected in order to extract a plurality of the density data. By phasing the extracted density data to superpose and average the extracted density data, the noise caused due to the members other than the rotary member to be corrected is decreased. Then, the density unevenness data for one period, which is caused due to the rotary member to be corrected, is obtained.
The process for extracting a plurality of density data for one period of the rotary member to be corrected from the obtained density data is generally carried out in accordance with the HP signal. For example, in an example of FIG. 12, the pattern is drawn for seven periods in synchronization with the HP signal, and is measured by the density sensor. Then, the head position of the pattern drawn on the recording medium is detected in accordance with the change in the output value of the density sensor, and the density data having the length corresponding to the seven rotations of the rotary member is obtained from the head position of the pattern. By equally dividing the obtained density data into 7 sections, 7 pieces of density data for one period of the rotary member, which are started from the same phase, are extracted. By averaging the extracted 7 pieces of density data and preparing the averaged density data for one period, it is possible to obtain the density data in which the components caused due to other members having the different periods are eliminated. In FIG. 12, by carrying out the FFT process and the inverse FFT process, the unrelated frequency components are eliminated. However, the above processes may be omitted.
In the averaged density data and the correction data for one period, which is prepared in accordance with the averaged density data, the head position thereof is coincident with the rising of the HP signal. Therefore, the correction process for correcting the density unevenness by using the above correction data is also carried out in synchronization with the rising of the HP signal.
As shown in FIG. 13, in case that the pattern is dense and the density difference between the pattern and the original color of the recording medium is large, it is possible to precisely judge the head position of the pattern in accordance with the change in the output value of the density sensor. On the other hand, as shown in FIG. 14, in case that the pattern is thin and the density difference between the pattern and the original color of the recording medium is small, it is difficult to judge the head position of the pattern in accordance with the change in the output value of the density sensor.
In Japanese Patent Application Publication No. 2007-140402, in order to avoid the above problem, as shown in FIG. 15, the technology in which the marker having the high density is necessarily provided on the head position of the pattern, is disclosed. Therefore, it is possible to detect the head position of the pattern by using the marker even though the pattern is thin.
According to the technology disclosed in Japanese Patent Application Publication No. 2007-140402, it is possible to easily detect the head position of the pattern. However, the density data for one period is extracted with reference to the head position of the pattern by using the theoretical length corresponding to the one period of the rotary member. For example, as shown in FIG. 15, in case that one period of the rotary member corresponds to the T line in the sub-scanning direction, which is the theoretical value, the density data for one period is extracted by each T line from the position of the marker.
However, as shown in FIG. 16, in case that the actual length of one period of the rotary member on the recording medium (T′ line) is different from the theoretical length (T line), even though the density data is extracted by using the theoretical length, the density data is not precisely extracted by one period. Therefore, even though the extracted density data are averaged, the noise caused due to the members other than the rotary member to be corrected cannot be precisely reduced.
As a factor which causes the actual length of one period of the rotary member on the recording medium to have a different value from the theoretical length, there is an error of the rotation rate of the rotary member. Further, there is a case in which the variable magnification is caused in the sub-scanning direction. For example, in case that the intermediate transfer belt is the recording medium, the difference between the actual length and the theoretical length is caused by the tension or the flexure of the intermediate transfer belt.
As shown in FIG. 17, instead of the extract of the density data using the theoretical length (T line), the HP signal is also supplied to the density sensor. In case that the density data output from the density sensor is extracted in accordance with the period of the HP signal, the shift caused due to the error of the rotation rate of the rotary member can be cancelled. However, even in the above method, as shown by the dashed line in the drawing, when the tension or the flexure of the intermediate transfer belt, the skew or the like is caused, the error caused due to this cannot be cancelled. Therefore, the density data cannot be extracted by one period with high precision.